Carbon black is a form of elemental carbon and it can be produced in varying particle sizes and structures. Structure is the degree of particle coalescence and agglomeration with a high structure carbon black having more particles agglomerated into random structures than does a low structure carbon black. Particle size and structure affect various qualities in rubber such as elongation, modulus, tear strength, tensile strength and resilience. Carbon blacks are used with both polar and nonpolar rubbers to enhance these qualities. Rubber stocks containing larger particle blacks display good resilience, but tend to be deficient in tensile strength and elongation. Conversely, rubber stocks containing small particle blacks display good tensile strength and elongation, but have poor resilience qualities. High structure carbon blacks improve the modulus and extrudability of rubber stock. The low structure blacks improve the resilience characteristics of rubber stocks. The carbon blacks in the middle ranges provide stocks that have average characteristics. Manufacturing techniques for carbon blacks have been perfected so that large or small particle carbon blacks with either high or low structure can be prepared.
Carbon blacks are graded by ASTM designation D 1765 standard classification. There are at least four types of carbon blacks, including furnace blacks, lamp blacks, thermal blacks and channel blacks. Furnace blacks are not modified to alter their inherent properties on rubber cure rates. Lamp blacks or lamp black substitutes have modified surfaces that affect the cure rate of rubber. Thermal blacks include very large particle blacks. These blacks are manufactured by incomplete combustion of natural gas. Channel blacks are also produced by incomplete combustion of natural gas. The carbon particles which are intermediate in size, are deposited on steel channels over the flame. The surface of the particle is not altered to affect rubber cure rate. The structure of blacks is defined by the n-Dibutyl Phthalate Absorption number (DBP number) and is measured by the D 2414 test method. The larger the DBP number the higher the structure. Particle size is defined by Iodine Adsorption Number and is measured by ASTM D 1510 test method. There is an inverse relationship between the iodine number and particle size; the higher the number the smaller the particle.
In manufacturing rubber products a carbon black with certain structure and particle size is used to enhance a certain desired characteristic while sacrificing others. For instance, a large particle black will be used to achieve desired compression characteristics at the expense of tear and modulus. Combinations of carbon blacks with the same size, but different structures have been used together. Using carbon blacks with the same size characteristics limits flexibility in varying qualities in the rubber products. However, utilizing carbon blacks with different sizes has generally been avoided because the mixture of different sizes of carbon blacks are deemed incompatible.
A rubber products manufacturer selects a carbon black from those with different size and structure to make a product with varying characteristics. Even so, utilizing one type of carbon black or pairs matched for size, limits the qualities achieved in the rubber, because the enhancement of certain qualities is detrimental to others. Recommended formulations for transmission belting for engines, hydraulic hose tubes and wire and cable jackets include mixtures of large particle carbon blacks to provide desired compression characteristics. The Vanderbilt Rubber Handbook, R. T. Vanderbilt Co., Inc.; Norwalk, Conn.; 13th Ed. (1990) pp. 653, 721. Other formulations for high hardness rubber mix furnace blacks with large particle thermal blacks. Id. at 770. Typical formulations for seals include a matched pair of large particle carbon blacks with high and low structure. Id. at 737. Mixtures of carbon blacks with similar size and structure are used in some applications. For example, two large particle and low structure carbon blacks have been recommended for a tire body formulation. Id. at 606. Tire treads, on the other hand, contain small particle carbon blacks. Id. at 604, 605. The large particle and low structure is desired for resilience qualities in the tire body. The tread utilizes small particles with better tensile strength.
The present invention is a unique pairing of furnace carbon blacks with disparate qualities to provide a system that can be manipulated to produce rubber with balanced optimum qualities. Specifically, the carbon black system is a mixture of a pair of furnace carbon blacks. The pairs are mismatched according to size and structure. One of the carbon blacks has a DBP number of less than 110 and the other has a DBP number of greater than 110 to provide a mix of structures. The pairs are also mismatched according to size with one of the pair with an iodine adsorption of greater than 110 and the other with less than 110. The two carbon blacks are used together with the ratio of one to another not to exceed 3:1. The ratio is varied according to the qualities desired. The use of the mismatched carbon black pairs mixed with rubber stock has produced an improved rubber product with good compression and resilience characteristics generally attributed to the use of large carbon black particles in a rubber while retaining the good elongation, modulus, tear and tensile strength attributable to the presence of small carbon black particles. Similarly there is a mix of structures with the mix of particle size. The use of the unique carbon black system produces a rubber composition with a combination of all the optimum qualities of the large and small particles and high and low structure. The combination of resilience as well as good elongation, modulus, tear and tensile strength has been attained.
The carbon black system also includes, in combination with the mismatched carbon blacks, a fumed silica and silane coupler. Fumed silica has a much smaller particle size than precipitated silica which is also used in rubber processing. Silica increases tear strength. Fumed silica is expensive and builds viscosity in the rubber. Silane couplers used in conjunction with silica decrease elongation, but sometimes can produce excessively high modulus elongation. Silane couplers are used in an effective amount with the fumed silica in ranges known to those skilled in the art. A typical ratio is 10:1 fumed silica to silane coupler. The fumed silica is used in a 12.5% to 50% by weight to the combined weight of the carbon black pair.
The carbon blacks can be selected according to the parameters of this invention to maximize the desired properties of polar and nonpolar rubbers. Different types of cures for the rubber can be used with the carbon black system of this invention including sulfur and sulfur donor cures depending on the rubber stock. The use of the unique carbon black pairs with silica/silane system allows the manufacturer to maintain a minimal stock of carbon blacks and produce rubber products with a range of qualities. The unique carbon black pairs eliminate the need to maintain a range of different size and structure carbon blacks for manufacturing purposes.
The use of the mismatched carbon black pairs with a fumed silica and silane coupler produces unique rubber compositions. The carbon black system is mixed with the selected rubber stock and cured. The carbon black mixture comprises from about 40 parts per hundred to about 80 parts per hundred of the selected rubber. The rubber compositions, both polar and nonpolar rubber, have as the carbon black constituent not only both high and low structure carbon blacks with DBP numbers greater than and less than 110 respectively, but also larger and small particle carbon blacks with Iodine Adsorption Numbers of less than and greater than 110 respectively. The use of the new carbon black system produces a new rubber composition as described herein.
Carbon blacks are given an ASTM designation with the letter "N" used to indicate typical furnace blacks that receive no special surface modification to alter their influence on the rate of cure. The second character in the nomenclature is a digit to designate particle size with lower digits indicating smaller particles. The third and fourth characters are assigned arbitrarily, however the full designation of each of the carbon blacks assigned by the ASTM describes the carbon black as to particle size and the structure. For example, one of the mismatched pairs is ASTM D1765 designations N231 a small particle (121 Iodine Adsorption No.) and low structure ( DBP No. 92) and N550 a large particle (43 Iodine Adsorption No.) and high structure (DBP No. 121). By varying the amount of N231 and N550 within the range of 3 parts N231 to 1 part N550 or 1 part N231 to 3 parts N550, rubber stocks can be provided with varying characteristics in acceptable ranges for different rubber products. Accordingly, the rubber products manufacturer can stock a limited number of carbon blacks and produce a variety of rubber stocks.